Apparatus for backfilling of trenches and for providing pipeline padding material are known in the art. An example of such apparatus is provided in U.S. Pat. No. 4,955,756 wherein the apparatus is suspended in cantilever fashion above the trench from a laterally projecting hitch mechanism secured to a crawler type tractor or the like which travels generally parallel to the trench.
More specifically, the apparatus described in U.S. Pat. No. 4,955,756 includes a main frame pivotally cantilevered on a hitch affixed to a side of a crawler tractor. The hitch extends transverse to the direction of travel of the tractor and the main frame is suspended thereby so as to extend parallel to the tractor travel direction. A hydraulic cylinder is provided to vary the attitude of the frame, that is, the relative angle or attack between the main frame and the hitch. The apparatus further includes a storage hopper carried by the main frame into which unclassified backfill material is loaded. The hopper mouth is typically provided with a grizzly which precludes large rock and oversize masses of clay fill from entering the hopper. An upwardly inclined, longitudinally extending conveyor transports the fill material from the bottom of the hopper to a higher elevation whereupon the fill is discharged from the conveyor and passes, preferably, through a comminuting or shredding station, onto a downwardly inclined, longitudinally extending vibrating screen. The speed of the conveyor and the attitude of the apparatus may be varied by an operator to maintain a substantially uniform flow of backfill onto the vibrating screen. The shredder mechanism is a rotary device which beats the fill material with flails, breaking up larger lumps into smaller lumps. The screen is double-decked in that it includes an upper and a lower screen element. Fine material of padding size falls through both screens into the trench and onto a previously laid pipe. Fill which fails to pass through the upper and/or lower screen elements travels over the screen elements into a chute and are directed rearwardly thereby into the trench and onto the padding fill.
Although suitable for its intended purposes, the apparatus disclosed in U.S. Pat. No. 4,955,756 requires the tractor to be fitted with a counterweight at the side thereof opposite the hitch to compensate for the imbalance created in the system by virtue of the cantilevered backfill apparatus.
Further, as it taught in U.S. Pat No. 4,955,756, subterranean pipelines are frequently supported above the trench floor by spaced (i.e., approximately every 15 feet) small piles of sand bags or the like, thereby allowing the padding machine to deposit padding material under, on the sides of, and over the newly laid pipe. There are occasions, however, where such support means may be unfit for practical use. This may be the case, for example, when the wall thickness of the steel or plastic pipe is relatively thin (thin walled pipe). In this situation, intermittent support of thin walled pipe may result, due to the pipe's own dead weight, in a slight flattening or egg shaping of the pipe at the individual support sites. A desirable alternative would be to provide a continuous bed layer of fine grained fill material which affords uniform, non-injurious pipe support throughout its length.
In this connection, the backfill apparatus of U.S. Pat. No. 4,955,756 is not readily usable for laying a trench bed layer consisting solely of fine grained material. This is because the unclassified backfill is separated by size and laid by the machine such that the finest materials would be placed first, i.e., on the floor of the trench, with progressively coarser materials being placed atop the fines. From a practical perspective, this means the pipe being laid would be directly supported by the coarsest fraction laid by the apparatus. Thus, if the pipe is one which is coated with a corrosion resistant material, such coating is apt to be scratched or damaged by the coarse grains which, in turn, may facilitate corrosive activity and, possibly, premature failure of the pipe.
U.S. Pat. No. 4,861,461 describes another apparatus for laying a padding layer of fine grained backfill material in a pipeline trench. The apparatus includes a frame which carries a storage hopper for receiving unclassified backfill material and a first conveyor for delivering the backfill material from the hopper to a double-deck vibrating screen for separating fine particulate matter (to be used as a pipeline bedding layer) from coarser fractions of the unclassified backfill material, which coarser fractions are discharged from the apparatus via a second conveyor. In addition, the apparatus carries a third extendable and retractable conveyor for delivering to the pipeline trench the fines which pass the vibrating screen. In operation, the apparatus extends generally transverse to the trench and is vehicle-drawn or self-propelled generally parallel to the trench, whereby the third conveyor delivers the fines to the trench.
Although the apparatus is capable of laying a trench bed layer consisting of fine grained material derived from unclassified fill material, it is quite complex in construction. Furthermore, it has inherent limitations as to its operability in environments where local topography adjacent the trench is highly varied, particularly in elevation. That is to say, the height at which the fines conveyor is suspended above the ground is fixed and minor adjustments to its attitude are achieved by selective operation of short-throw hydraulic cylinders which pivot a turntable frame that carries the fines conveyor. Thus, the pipeline padding machine of U.S. Pat. No. 4,861,461 may be rendered essentially ineffective in situations where topography sharply rises or falls.
An advantage exists, therefore, for a trench backfill apparatus of uncomplicated construction that can lay a bed layer consisting of fine grained material derived from unclassified fill material, which apparatus would be operable to fill a trench without requiring its suspension thereover and be capable of functioning effectively in environments having highly varied local topography.